It is known from the prior art to determine the state of charge of a battery by means of sensors of current whereof the measured intensity is integrated for determining the remaining charge for each cell. This method of measurement lacks accuracy in particular owing to measurement drift and bias after long periods of measurement. Moreover, the question also arises of initializing the integration and the accuracy of the sensor.
It is known from the prior art to determine the state of charge of a battery by means of voltage sensors which measure the voltage at the terminals of a battery cell. Indeed, when the cell does not deliver any current, this voltage measurement can be used to estimate an open circuit voltage from which the state of charge can be determined. This method is disrupted when a current flows through the battery.
It is also known from the prior art to determine the state of charge of a battery using current and voltage measurements. In this case, estimation by integration is corrected by the voltage measurements when the battery does not deliver any current, e.g. when the vehicle stops.
It is also possible to estimate the state of charge using models of cell behavior via dedicated estimation tools such as Kalman filtering, for example. These methods have the disadvantage of being complex and costly.
The determination of a state of charge from a measured or predicted open circuit voltage is known from patent application EP1384086. One drawback of this solution is that it requires the use of the battery to be interrupted, during discharging or during charging, in order to measure the voltage at the terminals thereof in open circuit.